The safeword is a geniusly simple concept: a word that allows one player to tell their partner(s) when a limit has been reached, in no uncertain terms. With all the faux resistance that can come up in an S/M scene (a bratty schoolgirl screaming "Stop it! Stop it!" as she is spanked, when that's the last thing she wants), it can be impossible to tell when No means No. A safeword removes the guesswork. Before play begins, partners agree on a word that's unlikely to be mistaken as part of role playing dialog. "No," "Stop," and "Don't" are obvious bad choices, while less common words like "Mercy," "Heel," or "Red" retain the connotation but won't blend in as easily. You can get real fancy and blurt out a totally unmistakable word like "Fargo!" but what you gain in obviousness you lose by possibly killing the mood. Regardless of what you choose, the meaning is the same: I want to stop. Now.